Ring laser angular rate sensors are well known. One example of a ring laser angular rate sensor is Hanse, et al U.S. Pat. No. 4,751,718, which is incorporated herein by reference thereto. Present day ring laser angular rate sensors, commonly known as ring laser gyros, include a thermally and mechanically stable laser block having a plurality of formed cavities for enclosing a gap. Mirrors are placed at the extremities of the cavities for reflecting laser beams and providing an optical close-looped path.
Associated with such sensors is a phenomenon called lock-in which has been recognized for some time in the prior art and has been solved by rotationally oscillating a sensor. Conventionally, a piezoelectric actuator is connected to a suspension system which causes the block of the sensor to oscillate angularly at the natural mechanical resonant frequency of the suspension system. This dither motion is superimposed upon the actual rotation of the sensor in inertial space. The prior art includes various approaches to recover inertial rotation data free from dither effects.
Dither suspension mechanisms in the prior art are typically directed toward dithering a single ring laser gyro with a single dedicated dither motor imbedded in the hub of each ring laser gyro. Examples of such systems include Stjern, et al. U.S. Pat. No. 4,653,918 and Wirt, et al. U.S. Pat. No. 4,349,183, as well as the Hanse ring laser gyro.
Dither motors for dithering multiple ring laser gyro beams within an integral ring laser gyro block are also known. For example, LeChevalier U.S. Pat. No. 3,503,688, entitled "Multiple Axis Laser Angular Rate Sensor", discloses a solid block support apparatus which supports three sets of counter rotating laser beams. A shaft and block are vibrated or dithered about the axis of the shaft by a single mechanical dithering means. Components of the dither are presented to each triangular path and all three of the triangles are simultaneously dithered.
U.S. Pat. No. 4,856,901 to Ferris, et al., entitled "Velocity Control System Using Piezoelectric Transducers", discloses a design strikingly similar to the dither concept used in the LeChevalier '688 patent. Three ring laser gyro paths integrated into a solid block are mechanically dithered by a single dither mechanism through a cube diagonal. The present invention, in contrast, is a device for simultaneously dithering three discrete ring laser gyros each of polygonal shape and does not require an integrated solid block construction as in the '688 patent. Thus, the present invention allows the construction of smaller ring laser gyro packages using discrete ring laser gyros. This avoids the complexities and expense of fabricating a single three gyro block.
Other art provides various dither schemes. U.S. Pat. No. 4,801,206, to Benoist, entitled "Simplified Ring Laser Gyroscope Dither Control and Method", is also directed to a dither mechanism. A dither flexor is mounted between a ring laser gyroscope body and a support such that the ring laser gyroscope may be dithered to have angular oscillations about a sensing axis.
U.S. Pat. No. 4,711,575, to Butler, entitled "Non-pendulous Counter-Balance Dither Mechanism For Laser Gyro", discloses a laser gyro dither mechanism utilizing a three spring suspension system.
U.S. Pat. No. 4,710,027 to Fersht, et al., entitled "Method and Apparatus for Mechanical Dither Stabilization Of A Laser Angular Sensor", discloses an apparatus whereby one, two, three or four dither mechanisms selectively may be simultaneously energized. The '027 patent is directed to individual dithering means connected to each of at least three ring laser gyros which are selectively energized. This is different from the present invention in that the present invention comprises a single dither mechanism which simultaneously dithers multiple ring laser gyros.
U.S. Pat. No. 4,653,918 to Stjern, et al., entitled "Low Q Body-Dithered Laser Gyro Assembly", discloses a low Q ring laser gyro dither motor assembly.
U.S. Pat. No. 4,597,667 to Curby, et al., entitled "Dither Controller For Ring Laser Angular Rotation Sensor", discloses a dithering apparatus which is inserted into the hub of a ring laser gyro. The mechanism includes a pickoff sensor to measure parameters relating the sensor body position to a support means.
U.S. Pat. No. 4,349,183 to Wirt, et al., entitled "Spring For A Ring Laser Gyro Dither Mechanism", discloses a flexure spring assembly for a ring laser gyro dither mechanism using flexure springs between the hub and the rim. Each spring is driven by four piezoelectric crystal wafers.
U.S. Pat. No. 4,309,107 to McMere, et al., entitled "Laser Gyro Dither Mechanism", discloses a laser gyro dither mechanism utilizing a three-spring suspension system.
The cluster dither mounting apparatus provided by the instant invention offers significant advantages over the prior art schemes. One advantage is that the invention allows the fabrication of inertial measurement units in a package volume which is smaller than the packaging volume generally available with prior art schemes. Further, there is only a single dither motor in the system hardware and software elements are simplified. As a result, the cost of such units built in accordance with the present invention is low. For example, the packaging volume in one application of the invention fits a 3.7 inch diameter.times.2 inch length.